I was diagnosed with endo about 6.5 years ago after years of awful period pain. The only way to get a definitive diagnosis is to have a surgical procedure called a laparoscopy. It often doesn't show up on ultrasounds etc.
You need to see a gyno ASAP to investigate further. It's very scary but I put it off for years and really suffered a lot.
The interesting thing about endo is how it affects everyone so differently depending on where you have it. So your symptoms would be completely different to others. I have a friend who had zero symptoms but was diagnosed after years of infertility. Hers was quite bad but didn't cause any issues with pain etc. Mine was on one of my ovaries and the ligaments that hold my uterus in place and I had a lot of terrible period pain for years.
Good luck, it's a really horrible disease. I think if men got it then they would have discovered some much better treatments :/